There are a number of existing devices which are intended to convert electrical energy into mechanical displacement. Typically useful in applications where relatively small displacements are required, electrical-to-mechanical actuators can, for example, be used to position components, provide to vibratory motion, and otherwise to convert electrical power into mechanical or acoustic power.
One presently used positioning device is the electromagnetic solenoid. These use a wire-wound coil to cause movement of a ferromagnetic plunger. Typically, solenoids are able to exert force in only one direction. Motion in the one direction can be applied over a relatively long stroke, but a spring or the like is required to move the plunger back to the starting position. Solenoid devices are thus limited to slow motion, and oscillatory motion is limited. Further, they are of relatively low efficiency since electrical energy is dissipated in the coil when the current is flowing, regardless of whether there is any movement of the plunger.
Currently, both electromagnetic coil and piezoelectric devices are used to convert electrical energy into oscillatory motion, and are used for making high efficiency loud speakers and microphones. However, they are limited to high frequency operation (typically greater than 1 kHz) and cannot be used to apply a static force. Magnetic coils are used in lower frequency loud speaker systems, but the loud speakers are typically very inefficient.
A third type of electrical-to-mechanical actuator uses a magnetostrictive material in combination with a magnetic coil and/or a permanent magnetic. See M. L. Spano, A. E. Clark and M. Wun-Fogle, Naval Surface Warfare Center, "Magnetostriction of TbDy Single Crystals Under Compressive Stress," IEEE Transactions on Magnetics, pp. 1751-3, Vol. 26, No. 5, 1989; and M. Goodfriend and C. Jones, "Application of a Magnetostrictive Alloy, Terfenol-D, to Direct Control of Hydraulic Valves," Paper no. 901581 presented at the SAE International Off-Highway & Powerplant Congress & Exhibition, Milwaukee, Wis., Sep. 10-13, 1990. A magnetostrictive material will change its dimensions when subjected to a magnetic field, but the strain is typically quite small. A typical such material is an alloy of Terbium (Tb), Dysprosium (Dy) and Iron (Fe) known as "Terfenol." TbDyFe has a maximum strain at room temperature of about 0.25% depending on the exact composition of the alloy; but because it is a brittle intermetallic, it is unable to withstand very high stresses. Relatively large and heavy (e.g., 4 in..times.4 in.) coils are normally needed to produce the high magnetic fields required by TbDyFe actuators; and such devices also suffer from several other disadvantages--dissipation of power in the coil due to the wire resistance and the need for a bias field, a maximum efficiency of about 60%, and the need for considerable active cooling.